It is frequently desirable to ascertain whether the air in or around a locality such as a factory or a laboratory contains gaseous material that might be harmful or dangerous to health, that might present a fire risk, or that might indicate the failure of equipment designed to contain the material in gaseous or liquid form, and much instrumentation exists to detect and measure such materials (which are known, in the art, as “gaseous analytes”; typical such analytes are petroleum chemicals—propane, butane, hexane and so on, and chlorinated and/or fluorinated hydrocarbons such as methylene chloride).
A typical gas sampling device usually incorporates a pump or fan to draw the gas to be sampled through a probe into the body of the device and there present it to, or through, a suitable gas sensor. Alternatively the gas to be sampled may be presented to, or through, a suitable gas sensor by diffusion without the assistance of a pump or fan.
One particular form of sensor utilises ion detection; the sampled gas is subjected to a physical treatment, such as photoionisation or flame ionisation, that causes any analytes therein to be ionised, and the formed ions are then drawn to an electrode so causing a minute electrical current to flow through the associated detector circuitry. Dependent on their application such sensors fall broadly into one or other of two different types: portable/roaming or fixed.
The portable or roaming gas monitor or detector is, as its name suggests, carried around a site testing for gas. It is particularly useful in tracing leaks of gaseous analytes from their containment means, and in roaming over large areas of an industrial site to ensure a particular analyte is not present in quantities liable to present some risk. Unfortunately, because the temperature of sampled gas varies as the detector is conveyed from place to place, the gas drawn into the monitor may on occasion experience a sufficient drop in temperature to cause dew or mist to form in the sampled gas drawn into the probe. This mist may then condense on the probe walls, and even on or within the gas sensor itself, resulting in false readings, and even in damage to or failure of the sampler. By way of example, such condensation can all too easily occur in consequence of conveying a portable gas monitor from an air-conditioned car to a humid out-of-doors environment.
The second type of sampling device is the “fixed” gas monitor; secured in place on site it continuously monitors levels of gaseous analytes at the chosen location, and triggers an alarm on a certain threshold analyte concentration being exceeded. Again, even with such a fixed monitor condensation can occur within or on the sensor, often as a result of temperature differences between the sampled gas and monitor, particularly where the gas is drawn from some different environment—for example, from an out-of-doors location to a room containing the monitoring equipment. And again, any formed mist may condense within the device, and on or within the sensor itself, resulting in false readings, and possibly damage to or failure of the monitor.
The invention is concerned with modifications to the device's internal structure intended to cope with the possibility of false readings caused by condensation and the like.
Gas detectors of the aforesaid ion-detection type usually have a walled (enclosed) sampling chamber within which the gaseous analytes are converted into ions (positively or negatively electrically-charged particles), forming a plasma. An electric field is applied across the chamber's cavity by means of two or more electrodes—a counter electrode at one voltage and a sensing electrode at a different, (usually effectively opposite) voltage—which are often a part of, or are contained within, the chamber walls. The ions are attracted through the gas to the appropriate electrodes, causing a signal current to flow, and this current is picked up, amplified, and displayed. Unfortunately, any contamination, such as condensation, on the surrounds of the chamber or the electrode supports and extending between the electrodes will provide an alternative route for electric current to flow between the electrodes, and if this happens then an increased but spurious signal is obtained from them which can obscure the small ion-derived current signal.
Although of course the problem of condensation can be tackled by using filters and water-absorbent materials in an attempt to remove any water vapour from the sampled air, this is often simply not practicable. Moreover, in some environments the capacity of the available filters and drying agents can readily be exceeded by the cumulative condensation of water from an admitted sample gas stream. In addition, such filters and agents frequently remove the sought-after analytes themselves from the gas sample before they ever reach the gas sample chamber.
Accordingly, a need exists to overcome the problems of mist and condensation with prior art ionization devices